We have demonstrated that human platelets are chemotactic toward their physiological substrate, collagen, by three independent methods: 1) a procedure involving migration from a capillary tube; 2) an original technique involving the use of a 'micromaze'; and 3) chemotaxis as observed in a modified Boyden chamber. The phenomenon of platelet migration has an absolute requirement for intact platelet metabolism, functional microfilament proteins, and plasma membrane integrity. Preliminary experiments in our laboratory have indicated that when collagen is incubated in plasma in a modified Boyden chamber, platelet migration is stimulated in the opposite chamber. We have postulated that a low molecular weight substance was elaborated from the collagen (cytotaxin) was responsible for the migratory phenomenon. The experiments proposed in this application are directed towards: 1) specifically identifying the cytotaxin(s) and the cytotaxigen(s) that may be produced when blood vessels are damaged; 2) isolating and characterizing chemoreceptors and chemosensors that may be present on the platelet plasma membrane; 3) establishing a relationship between the chemoreceptors and the contractile proteins in platelets; 4) studying the role of calcium in the migration reaction; and 5) attempting to distinguish molecules which influence the rate of migration from those which have an effect on the direction of migration. The underlying goal of this research is to gain further knowledge of platelet-platelet and platelet-blood vessel interactions.